The aim of the research proposed in this grant is to gain insight into the action of recessive oncogenes in cancer. Several approaches have been applied to identify genetic elements involved in turmorigenesis. Oncogenes were initially defined in tumor-inducing retroviruses and in tumor DNA capable of transforming nonneoplastic cells in culture. Most onocogenes are activated versions of proto-oncogenes that exist in normal cells. Another class of cancer genes has been proposed for which loss of gene function is associated with oncogenesis. The involvement of such genes has been suggested for many tumor types, e.g. retinoblastoma, breast cancer, colon cancer, renal cell carcinoma, etc. Since tumor formation is associated with loss of gene function instead of activation, the latter class of genes have been termed "recessive oncogenes" or cancer suppressor genes". The retinoblastoma susceptibility (RB) gene is the only human recessive onocogene cloned to date. RB gene is localized in chromosome band 13q14. We and other have found mutational inactivation of the RB gene not only in retinoblastomas but also in a significant percentage of osteosarcomas and soft tissue sarcomas. The presence of a tumor suppressor gene on chromosome 13 for some breast caners has been suggested. Studies in Drosophila also suggest the association of 24 loci with tissue-specific tumors. Inactivation of both alleles of the lethal(2)giant larvae (l(2)gl) gene causes malignant neuroblastoma in the larval brain and tumors of the imaginal discs. Moreover, developmentally regulated expression of l(2)gl is important for normal growth. This proposal focuses on two aspects of RB function: (1) RB gene involvement in the genesis of breast cancer and (2) a developmental role for the RB gene using the fruit fly Drosophila as a model. I. Function of the RB gene in breast cancer. We have detected mutation of the RB gene in two outy of nine breast tumor cell lines. Priminary breast tumors will be stuied for RB gene inactivation. By retrovirus mediated gene transfer we will introduce normal RB gene into breast tumor cell lines lacking RB function to test for its tumor suppression activity invivo and reversibility of transformed phenotype in vitro. In addition, modulation of receptors for transforming growth factor and epidermal growth factor by the RB gene will be explored. II. The RB gene homolog in Drosophila. We will use the Drosophila system as a model to study the development expression and function of RB. Our preliminary data suggests the presence of a RB homolog in this organism. Genomic and cDNA clones of the Drosophila RB gene (dRB) will be extensively characterized. In addition, dRB wil be localized to polytene chromosomes. Classical genetic procedures will be used to identify putative dRB mutants. We will then use P-element mediated germ line transformation to study the dRB gene in its normal environment.